The invention refers to a flatbed printing machine with a printing table.
Particularly in the case of digital printing machines, high standards are imposed on the flatness of a printing table and on the medium to be printed. This is the case, for instance, because when using ink-jet heads, the heads must be held at a constant gap of about one millimeter above the medium to be printed. In conventional printing machines, vacuum holes are provided in the printing table in order to suck the medium to be printed against the surface of the printing table and thus prevent formation of creases on the medium to be printed.
With the invention, a flatbed printing machine shall be created which ensures that the medium to be printed lies on the printing table with its full-surface.
According to the invention a flatbed printing machine with a printing table and a pressing device for the medium to be printed is provided, wherein the pressing device exhibits parallel guide rails on both sides of the printing table, by which a movable pressing rail is guided along the printing table.
By providing such a pressing device, the medium to be printed can be pressed against the printing table, by use of the pressing rail and especially any air between the medium to be printed and the surface of the printing table can be swept out by the motion of the pressing rail above the printing table. Especially in connection with sucking the medium to be printed against the printing table surface, it can be ensured that the medium to be printed lies on the full surface on the printing table.
In a further embodiment of the invention the pressing rail is provided with a brush or a roller for pressing of the printing medium.
It is possible to smooth a medium to be printed both by using either a brush or a roller. The type of the medium to be printed, for instance, paper or film can be decisive for the choice of either a roller or a brush. In the case of a roller, it can roll passively and therefore a separate drive in not necessary.
In a further embodiment of the invention, the pressing rail may be lowered or raised by a lifting device on the printing table.
These measures allow the medium to be printed to be automatically smoothed and, furthermore, it can be ensured that when printing sheets of smaller size compared to the size of the printing table, it is not necessary to smoothen the entire surface. The lifting device features, for example, swiveling arms actuated by way of lift cylinders.
In a further embodiment of the invention, the printing table is provided with several vacuum holes leading to the printing table surface.
Such vacuum holes can, for instance, run perpendicular to the printing table and intersperse it. Suction applied to the vacuum holes can reliably hold the medium to be printed during the printing process. Any creases in the medium to be printed can then be smoothened by way of the pressing device according to the invention.
In a further embodiment of the invention, means are provided for applying suction to the vacuum holes, and may be controlled such that suction is then applied to a vacuum hole during the period when the pressing rail passes over the hole.
In this manner, smoothing of the medium is simplified, since the suction, for instance, is only applied when the pressing rail has passed over the respective vacuum hole. The result is that the medium to be printed is only sucked against the printing table surface when the respective area has already been traversed by the pressing rail, and thus already swept over and smoothened. In this manner, the process of smoothing out any creases in the medium to be printed, and at the same time sweeping out any air bubbles trapped between the medium to be printed and printing table surface, is not obstructed by adherence of the medium to be printed due to vacuum pressure. Since the motion of the pressing rail and the application of suction are adjusted to match one another, even in the case of comparatively low pressing pressure of the pressing rail, it is ensured that any creases on the medium to be printed are smoothened after the pressing rail has traversed above the medium to be printed.
In a further embodiment of the invention, the vacuum holes are arranged in rows on the printing table, parallel to the pressing rail, whereby a row of vacuum holes or a number of rows of vacuum holes assigned to this particular section are subjected to vacuum pressure.
Such a grid arrangement of vacuum holes simplifies their control. After the pressing rail has passed over a row of vacuum holes, suction remains active on this row in order to prevent the medium to be printed from shifting its position on the printing table during further movement of the pressing rail. Suction remains active also during the subsequent printing process.
Further features and advantages of the invention may be derived from both the claims and the following description of a preferred embodiment of the invention in connection with the drawings. The following are depicted in the drawings.